In any explosion, a certain amount of protection from blast may be
gained by having any large and substantial object between the protected
object and the center of the explosion. This shielding effect was
noticeable in the atomic explosions, just as in ordinary cases,
although the magnitude of the explosions and the fact that they
occurred at a considerable height in the air caused marked differences
from the shielding which would have characterized ordinary bomb
explosions.
The outstanding example of shielding was that afforded by the hills in
the city of Nagasaki; it was the shielding of these hills which
resulted in the smaller area of devastation in Nagasaki despite the
fact that the bomb used there was not less powerful. The hills gave
effective shielding only at such distances from the center of explosion
that the blast pressure was becoming critical--that is, was only barely
sufficient to cause collapse--for the structure. Houses built in
ravines in Nagasaki pointing well away from the center of the explosion
survived without damage, but others at similar distances in ravines
pointing toward the center of explosion were greatly damaged. In the
north of Nagasaki there was a small hamlet about 8,000 feet from the
center of explosion; one could see a distinctive variation in the
intensity of damage across the hamlet, corresponding with the shadows
thrown by a sharp hill.
The best example of shielding by a hill was southeast of the center of
explosion in Nagasaki. The damage at 8,000 feet from X consisted of
light plaster damage and destruction of about half the windows. These
buildings were of European type and were on the reverse side of a steep
hill. At the same distance to the south-southeast the damage was
considerably greater, i.e., all windows and frames, doors, were damaged
and heavy plaster damage and cracks in the brick work also appeared.
The contrast may be illustrated also by the fact that at the Nagasaki
Prefectural office at 10,800 feet the damage was bad enough for the
building to be evacuated, while at the Nagasaki Normal School to which
the Prefectural office had been moved, at the same distance, the damage
was comparatively light.
Because of the height of the bursts no evidence was expected of the
shielding of one building by another, at least up to a considerable
radius. It was in fact difficult to find any evidence at any distance
of such shielding. There appeared to have been a little shielding of
the building behind the Administration Building of the Torpedo Works in
Nagasaki, but the benefits were very slight. There was also some
evidence that the group of buildings comprising the Medical School in
Nagasaki did afford each other mutual protection. On the whole,
however, shielding of one building by another was not noticeable.
There was one other peculiar type of shielding, best exhibited by the
workers' houses to the north of the torpedo plant in Nagasaki. These
were 6,000 to 7,000 feet north of X. The damage to these houses was
not nearly as bad as those over a thousand feet farther away from the
center of explosion. It seemed as though the great destruction caused
in the torpedo plant had weakened the blast a little, and the full
power was not restored for another 1,000 feet or more.